This paper provides a short and transparent solution for the covering cost of
white-grey trees which play a crucial role in the algorithm of Bergeron {\it et
al.}\ to compute the rearrangement distance between two multichromosomal
genomes in linear time ({\it Theor. Comput. Sci.}, 410:5300-5316, 2009). In the
process it introduces a new {\em center} notion for trees, which seems to be
interesting on its own.